There are many types of color blindness, caused by the dysfunction of
certain photoreceptors in the retina. Among the colors and color distinctions
that individuals may find difficult or impossible to perceive are: red, green, blue, red-green, blue-green, red-blue, and red-green-blue (total color blindness). Males are ten times more likely to be color blind than females. The disability is especially pronounced among male caucasians: 8% suffer some form of red-green color blindness. For color distinctions to be perceived, it is
important to vary as many primary colors as possible. Perception of color contrast also decreases with age: brighter colors are better for older readers.